Let's just get it out of the way: Windows Phone 7 is the most exciting thing to happen to phones in a long time.

There are a million reasons why Windows Phone 7 matters. It's the most important PC company in the world, battered, bruised and badly lagging, coming back to the next generation of PCs, after crashing on a bunch of rocks and abandoning ship. It's potentially the most tectonic shift in mobile since the launch of iPhone and Android. It's Microsoft starting over and betting massively on its future. It's a very different kind of Microsoft product. It could be the beginning of something truly great.

Windows Phone 7 is the most aggressively different, fresh approach to a phone interface since the iPhone. Everything is superflat and two dimensional. Ultra-basic squares, primary colors and lists. Fonts are gigantic and clean, white text on an almost universally black void. It's fluid. This spartan nature is emblematic of the entire OS, for better and for worse. You don't get a lot of choices; there are no custom ringtones, for instance. It just is how it is. And while it looks and feels very different in some regards, it's still uncanny just how deeply inspired Windows Phone is by the iPhone in its philosophy, versus anything else Microsoft or anybody else has made.

The interface is oriented around three core concepts:

• Hubs Essentially panoramic apps that span multiple screens. Ironically, what really proves the Hub concept works are the third-party apps that use it. It works perfectly with Twitter, Netflix, Foursquare and Facebook, swiping over a screen to get to mentions, or to see your friends checkins.
• Live Tiles These are the home screen's icons, and they update with fresh info like email counts. But the Tiles don't do quite enough to function as full-fledged widgets—the weather Tile, for instance, doesn't show the weather.
• The App Bar a semi-persistent menu/taskbar that hides deeper actions—like starting a new email or switching tabs in Internet Explorer. It's a necessarily evil, given how radically Microsoft has reduced the onscreen UI.

Windows Phone strikes the best balance of any smartphone between web-oriented and local storage, using the cloud for info like contacts and apps, tying itself to a PC (or Mac, with a basic client) only for big updates, music and video syncing. Contacts from Facebook and Google beam in, sync and integrate perfectly. Finding your lost phone, photo uploading and note syncing is built-in, automatic and free.

The core OS is very, very good, if embryonic in some ways, much like the original iPhone. It's really up to the apps to make Windows Phone usable. Microsoft won't have 250,000 at launch, but true to their word, it seems like they'll have a lot of what's needed, with the early launch apps feeling great. That said, it's still way too tricky to find things, especially given there ain't that much in the store yet. It could be the best platform launch yet.

The most polished UI this side of the iPhone: It just feels amazing, even if it is missing some things like copy and paste (coming next year, supposedly). A nearly perfect melange of Microsoft services—Bing (Maps and Search), Zune, Xbox Live, Office—in a cohesive, logical and typically beautiful way. (Though the more tied into Microsoft you are, the better experience you'll have, like Google and Android.) Native apps are almost gratuitously tasty eye candy. The keyboard is boss. Outlook mail app looks and works fantastically. Zune streams over the air. Even IE doesn't suck, though anything that renders poorly in desktop IE will also do so on the phone.

Remember how iTunes wasn't so bad, and then Apple kept pinning on feature after feature, bloating it into a massive, disgusting corpus? Yeah, well, the Zune desktop client is slowly meeting the same fate now that's it used to sync your phone and as the browser for the app marketplace.

And in pursuit of a stark aesthetic, there's a radical reduction of elements on a WP7 screen: The home screen, for example, only fits eight tiles at once. So to get to something else, you've gotta swipe a mile or two. (The iPhone gives you access to 20 items; Android 2.2, up to 19.) No universal search to call up apps. No singular email app: Every email account creates its own tile, which sucks when real estate is so valuable. Browsing for apps is the single most painful experience of Windows Phone. Stumbling upon Netflix felt like a happy accident after 10 minutes of flipping through apps to see what was new. Loading app lists takes forever, and it's the one time the phone ever becomes totally nonresponsive. Annoying "Resuming..." screen pops up any time you lock the phone with an app open and then turn the screen back on. Sometimes the whole app reloads. It's highly annoying, like hitting a speed bump in a Ferrari. No multi-tasking.

Windows Phone 7 is really great. A solid foundation, it's elegant and joyful. True, a lot of that greatness is potential. But if anybody can follow through on their platform, it's Microsoft. Should you buy this instead of an iPhone or Android phone though? In six months, after the ecosystem has filled out, the answer will be more clear. But right now, Window Phone is definitely an option. Considering where Microsoft was just a year ago, that's saying a hell of a lot.